The present invention relates generally to positioning and scanning mechanisms for driving optical elements, and more particularly to an apparatus and method for rotating an optical element, such as a diffraction grating, to a desired position, to a set of desired positions or over a range of selected positions.
Scientific instruments, such as spectrophotometers used for liquid chromatography or capillary electrophoresis, typically analyze samples by detecting the absorbency (or transparency) or fluorescence of the samples to electromagnetic radiation, such as visible light. The light may be of one or more wavelengths or a band of wavelengths. Generally, the desired wavelengths of light are produced by projecting light, which may be generated by a broadband light source, onto a dispersing element, such as a rotatable diffraction grating of a monochromator. The grating and, possibly, additional optical elements direct the light to the sample or target of interest. The angular displacement of the diffraction grating relative to the incoming light beam can be closely correlated with the individual wavelengths or range of wavelengths at which the sample is to be analyzed. By controlling the angular rotation and position of the diffraction grating, a range of wavelengths can be scanned at a known rate over a known time interval and, consequently, the individual wavelengths can then be distinguished as a function of time.
Prior art monochromators are known which employ a stepper motor or a moving-iron galvanometer to generate and regulate the angular rotation of the diffraction grating. For example, U.S. Pat. No. 4,211,486 issued to Magnussen, Jr. et al. discloses a spectrophotometer utilizing a closed loop servo positioning mechanism to control the angular positioning of the diffraction grating. A feedback control circuit senses changes in capacitance between a rotating diffraction grating armature and a set of fixed electrodes to generate a control signal. The control signal is then compared to a predetermined position setting and the differential error signal is used to control the diffraction grating.
Further prior art systems are known which incorporate position sensing devices for detecting and controlling the angular position of an optical element, such as a diffraction grating. For example, an article by Grenda et al., "Closing the Loop on Galvo Scanners", EOSD Magazine, April 1974 discloses two grating position sensing systems using auxiliary light sources. The first system uses a light emitting diode (LED) to produce a light beam which is reflected by a diffraction grating onto a position sensing silicon photodetector which generates an electrical signal representative of the position of the diffraction grating. A feedback control system uses the generated electrical signal to regulate the angular rotation and position of the diffraction grating.
Grenda et al. also disclose a second position sensing device which uses an auxiliary light source to produce a light beam reflected by the diffraction grating toward a stationary transparent grating. The stationary grating is etched with multiple lines indicating changes in position of the diffraction grating. A photodetector counts the number of light pulses received from the transparent grating to determine the position of the diffraction grating.
U.S. Pat. No. 4,804,266 discloses a rapid-scan spectrophotometer using an optical incremental encoder to control data acquisition and differentiate between wavelengths. A floppy disk drive motor or tape drive motor continuously rotates the grating at a substantially constant velocity. Since the grating is rotated at a substantially constant velocity, wavelengths present in the time intervals between encoder pulses can be determined by interpolation.
The two major operative performance parameters for a scanning mechanism are speed and precision. The prior art systems have experienced problems in providing the necessary speed and precision. Stepper motors have typically been slow and imprecise. Moving-iron galvanometers frequently experience eddy current and magnetic hysteresis causing precision errors. Furthermore, moving-iron galvanometer scanners are very sensitive to ambient conditions. In order to compensate for the above defects, expensive and complex electronic control systems have been designed. Nonetheless, the performance and repeatability of the prior art systems continue to be problems in the art.
Accordingly, the need exists in the art for an improved apparatus and method for rotating an optical element, such as a diffraction grating, which reduces manufacturing costs, provides improved repeatability and precision, and is substantially insensitive to fluctuating ambient conditions.